NO. 1985. NEW SPECIES OF SILURIAN FOSSILS— WILLIAMS. 331 



The second species is much more transversely elongate and has the 

 anterior ear distinctly rounded without reentrant curve to the ante- 

 rior border; for this form I propose the specific name transversalis. 



Reference also is made, in the comments upon the species after its 

 description, to a "rotundato-quadrate form." This is probably a 

 representative of the form I am describing as a new species under the 

 name Palseopecten cobscooki, making it the type of the new genus 

 Palxopeden, which appears to include all the forms gathered under 

 the name Avicula? danbyi by M'Coy. 



PELECYPODA. 



PAL^^OPECTEN, ne^A^ genus. 



The genus combines certain characters which, in later geologic 

 times are found separated in different genera and distributed in the 

 separate famihes Pectinidae and Aviculidse of modern writers. The 

 shell is inequivalve and inequilateral. In the type species the shell 

 is nearly symmetrical as in typical Pectens; in other species, inti- 

 mately associated with it by intermediate forms, the shell is conspic- 

 uously inequilateral, the posterior body of the shell being much pro- 

 duced as in the genus Follmamella. Anterior ear and posterior wing 

 both developed, the posterior wing larger than the anterior ear, both 

 flattened, and no distinct evidence of byssal sinus. Cardinal border 

 straight and shorter than transverse diameter of body of shell. Left 

 valve gently convex, beak low, inconspicuous, within the middle 

 third of the cardinal border. Right valve flattened, sUghtly convex 

 transversely across the umbonal region but distinctly concave (dish- 

 ing) from beak to front margin. Ligamental area of hinge distinctly 

 marked by fine longitudinal lines; and under the beak of the left 

 valve a distinct triangular cartilage pit. Two strong diverging 

 ridges are seen, one each side the beak of the left valve, of about 5 to 

 8 mm. length in the type species (crural ridges) . In the transverse 

 species these crural ridges are less strong but longer than in the type 

 species. The shell substance was apparently thin; the surface lines 

 and foldings show with almost equal distinctness in interior molds 

 and on exterior surfaces. These surface markings consist of fine 

 linear concentric strise and rather broad, irregular concentric foldings 

 crossed on the left valve by obtuse ridges radiating from the beak and 

 generally becoming obsolete toward the margin. Over the umbonal 

 region the radiating ridges are separated by flattened furrows little 

 wider than the ridges; but toward the front the furrows become sev- 

 eral times the width of the ridges, and occasionally a secondary ridge 

 develops between two adjacent primary radii. In size the specimens 

 vary from 1 to 3^ inches in transverse diameter. 

 Type-s'pecies. — Palxopecten cobscooki, new species. 



